The subject of the present invention are a system and a method of optical telecommunication, which is particularly adapted for transmission with wavelength division multiplexing, or WDM, in which the various signals are recognized and separated on reception.
In transmission by wavelength division multiplexing, or WDM, it is required to send several mutually independent transmission signals down the same line, consisting of optical fibers, using optical wavelength domain multiplexing; the signals transmitted can be either digital or analog and are distinguished from one another since each possesses a specific wavelength separate from that of the other signals.
The carrying out of such WDM transmission makes provision for allocating specific wavelength bands of preset width, hereinafter termed channels, to each of the signals with different wavelength.
Such signals, distinguished hereinafter by a wavelength value, the said central wavelength of the signal, have some spectral width about the central wavelength value, which depends, in particular, on the characteristics of the laser source for the signal and on the modulation imparted thereto in order to associate information with the signal. Typical values of spectral width of the signal emitted by a laser, in the absence of modulation, are about 10 Mhz; in the presence of external modulation, for example at 2.5 Gbit/s, it has a spectral width of around 5 GHz.
For the purpose of transmitting signals in a large number of channels, making use of the so-called third transmission window of silica fibers and of the useful band of optical amplifiers (typically from 1535 to 1561 nm), the wavelength separation between these signals beneficially is of the order of nanometers.
For correct reception of such transmission signals, it is therefore necessary to effect a separation between these signals, in order to forward them to the respective users.
Narrow-band optical filters can be used for this purpose, through which only the signal in the selected channel can pass, thereby guaranteeing the absence of unwanted signals, which would constitute noise if super-imposed on the selected signal. The use of such filters, however, requires both high wavelength-stability of the signal transmitted, and high intrinsic stability of the central wavelength of these filters. Furthermore, in the case in which the number of channels is large, the passband of the filters has to be sufficiently narrow.
Filters having narrow passband and high isolation between one channel and another are difficult to obtain commercially, in particular on account of problems of industrial reproducibility.
Patent Application EP 0 629 885 proposes to use, in each selective channel filter, two Bragg reflection filters placed in series. The passband is obtained by suitably positioning the two reflection peaks of the filters.
Patent U.S. 5,504,609 describes a demultiplexer for selecting a particular channel from the multiplexed signal and delivering it to the receiver. To carry out channel selection, the multiplexed signal is sent to an optical filter via a coupler. The wavelength of the channel is reflected by the optical filter to the receiver via the coupler. The optical filter comprises a Bragg-grating element which reflects the wavelength of the channel and transmits all the others.
Patent Application EP 0 713 110 describes the use of a filter consisting of a fiber which incorporates a Bragg-grating normally reflecting filter and of two inclined Bragg filter gratings. This fiber is connected to a port of an optical circulator.
The Patent abstract of Japan, vol. 096, No. 011, 29 November 1996, JP 08 184730 A, discloses a device that is provided with optical filters which have transmission wavelength bands corresponding to the respective wavelengths of light signals constituting a wavelength multiplex light signal and transmit only the light signals in the transmission wavelength bands while reflecting others and plural mirrors which are provided corresponding to mutually adjacent optical filters and remove light signals belonging to transmission wavelength band of one optical filter from the light signal reflected by the filter and reflects other light signals to the other optical filter. For demultiplexing, when part of a light signal which should be transmitted originally through each optical filter is reflected by the optical filter, the light signal is adsorbed by a mirror to prevent interference.
The article of O. E. Delange, with the title "Wide band optical communication systems: Part II--Frequency-Division multiplexing", proceedings of the IEEE, Oct. 1970, vol. 58, no. 10, discloses that the composite multicarrier incoming signal is applied to the first analyzer, which is followed by a quarter-wave plate and a filter tuned to pass the first channel carrier and its sidebands. Ideally all remaining applied frequencies are reflected back to the analyzer, which passes them out of its sidearm and onto a mirror that reflects them to the second analyzer where the process is repeated.
The Patent abstract of Japan, vol. 096, no. 011, 29 November 1996, JP 08 172395 A, discloses a wavelength separating device where plural fiber grating filters different by reflection wavelength and optical circulator are cascaded.
The Patent U.S. 5,457,760, discloses that in the demultiplexer, branching waveguides are positioned between the input waveguide and N output waveguides. These branching waveguides have optical filtering elements, such as Bragg gratings, formed therein. Each output optical waveguides having an optical filter comprising an array of optical filtering elements, such as Bragg diffraction gratings, formed in a contiguous portion of the output waveguide.